DE 41 12 018 A1 describes a classifier for the separation of granular bulk products into at least three fractions, the system incorporating in a vertical housing several classifying separator wheels concentrically interacting at a distance from one another. In that classifier system, the separating air stream passes through one classifying wheel after the other and each wheel has its own product separation parameters. In a derivative design version, the wheels are mounted in separate positions within the housing, each with its own independent drive. In a classifier of that type, the first separation takes place in the first separation stage at the outermost classifying wheel having the largest diameter, whereby the fine and intermediate fractions are separated from the coarse fraction. Any further fine fraction is obtained only from the fine and intermediate fractions derived from the first separation stage, meaning that the coarse fraction is exposed to only one single separation stage. To obtain high-quality coarse material, the coarse product requires intensive screening.
A coarse product fraction free of fine particles and thus free of dust is typically obtained by means of a classifier which permits intensive and nearly total separation of the bulk material and thus of the coarse fraction.
The air classifier described in the German patent DE 40 40 890 C1 employs an optimized level of effectiveness in separating the bulk-material feed into at least three streams of particles differentiated by grain size. It incorporates two concentrically interleaved separator wheels with blades which are large in axial direction. Due to this design concept, the bulk material remains in the separating zone over an extended distance, permitting nearly all fine and intermediate particles to be separated. But in this case as well, any further fine fractions are extracted only from the fine and intermediate fractions derived from the first separating stage. There is no second follow-up screening of the coarse material.
The European patent EP 0 244 744 B1 describes a centrifugal-force-operated classifier with an essentially cube-shaped housing containing several individually driven separator wheels each of which connects to a separate air/fine-fraction outlet The stated purpose of the separator wheels, arranged in a parallel, series or over-under configuration, is to provide a high-performance classifier capable of simultaneously delivering several different granular fractions.
With that classifier, the bulk material can be consecutively fed to several separator wheels, permitting repeated processing of the coarse fraction. However, since the separator wheels are always juxtapositioned at a relatively short distance from one another, they affect one another's performance.
This entails significant drawbacks. For example, not every one of the separator wheels has its own individual air intake, a fact that complicates the precise setting of the fractionating boundary of each individual classifying wheel. Due to the spatial, co-planar setup of the separator wheels and the cramped configuration it is possible for coarse material that is rejected by one of the wheels and radially hurled outwards to penetrate into the adjoining wheel in undesirable fashion, being interspersed with the fine-grain fraction of that wheel even though by virtue of the preset fractionating boundary of that wheel it should have been rejected. This design thus causes the fine and intermediate fractions to display an undesirably high content of coarse particles (sputter).
These drawbacks can be avoided by using a classifier per DE 39 24 826 A1 by positioning the wheels one above the other, suitably spaced apart A classifier of that type permits the separation of granular bulk material into at least three fractions. It consists of an essentially vertical housing with exit ports for the fine, intermediate and coarse material, with each separating wheel having its own air intake and its own conduit for exhausting the separating air stream.
Obtaining multi-stage separation of fine and coarse particles, each within tight grain-size tolerances by means of very crisp separation of the individual sizes, typically involves the use of several separate classifiers process-linked for fluidized or nonfluidized operation in a processing system by means of feeder systems such as pipelines. During the movement from one classifier to the next, complete redispersion of the particles in the air can occur. This is in fact necessary in order to establish optimal conditions in the downstream classifier for complete separation of the material.
Where several separating stages are combined in one housing, particle dispersion in the pipelines is no longer possible, requiring some other solution for dispersing the material between two separating stages. Per DE 39 24 826 A1, the particles to be separated would be circulated and loosened up in the transition space between the first separating stage and the second stage below it by making that transition space funnel-shaped. In enhanced designs, that funnel-shaped transition space would contain centrifugal fliers or centrifugal disk vanes serving to deagglomerate and/or break down the material.
Highly wear-resistant baffles mounted on the inside wall of the housing serve to protect the transition space against excessive wear. Unfortunately, apart from and due to the desired deglomeration, the high-velocity impact of the particles impinging on the inner housing wall leads to the formation of deposits and reagglomeration, especially when the inner housing wall is bombarded with an increased number of particles in the case of inadequate or altogether absent provisions for moving the particles away from the inner housing wall. For very sensitive products that need to be separated but should not yield only fine particles as the final fraction, for instance toners, an additional size reduction of the granules during the separation process is not desirable.